Non-uniformly polished scintillation crystal for a gamma camera

ABSTRACT

A gamma camera is provided having an array of photodetectors and associated circuitry for detecting and converting light energy to electrical energy. The gamma camera further includes a scintillation crystal positioned in proximity to the array of photodetectors for detecting gamma photon emissions and generating light energy. At least one portion of at least one surface of the scintillation crystal is polished differently than at least another portion for yielding a substantially different light response function for the generated light energy. That is, the scintillation crystal is non-uniformly polished or has a non-uniform level of smoothness. The non-uniformly polished scintillation crystal improves signal-to-noise ratio and image quality with respect to spatial resolution.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to nuclear medicine, and a gammacamera for obtaining nuclear medicine images of a patient's body organsof interest. In particular, the present invention relates to a gammacamera for obtaining nuclear medicine images by detecting radiationevents emanating from a patient and having a non-uniformly polishedscintillation crystal capable of providing at least two different lightresponse functions.

2. Description of the Related Art

Nuclear medicine is a unique medical specialty wherein radiation is usedto acquire images which show the function and anatomy of organs, bonesor tissues of the body. Radiopharmaceuticals are introduced into thebody, either by injection or ingestion, and are attracted to specificorgans, bones or tissues of interest. Such radiopharmaceuticals producegamma photon emissions which emanate from the body.

Conventional gamma cameras utilize a scintillation crystal (usually madeof thallium-activated sodium iodide (Nal(TI))) which absorbs the gammaphoton emissions and emits light photons (or light events) in responseto the gamma absorption. An array of photodetectors, such asphotomultiplier tubes, is positioned adjacent to the scintillationcrystal. The photomultiplier tubes receive the light photons from thescintillation crystal and produce electrical signals having amplitudescorresponding to the amount of light photons received. The electricalsignals from the photomultiplier tubes are applied to position computingcircuitry, wherein the location of the light event is determined, andthe event location is then stored in a memory, from which an image ofthe radiation field can be displayed or printed.

FIG. 1 illustrates a gamma camera 10 comprising a Nal(TI) scintillationcrystal 12. Generally, scintillation crystal 12 is large enough (10×10cm) to image a significant part of the human body. An array ofphotodetectors 13, such as an array of photo-multiplier tubes (PMTs)having a plurality of PMTs 14, views scintillation crystal 12, to givepositional sensitivity. Each PMT 14 has an X and a Y coordinate. When aphoton is absorbed by scintillation crystal 12, light energy isgenerated in the form of visible light. A number of PMTs 14 receive thelight via a respective light guide 16 and produce signals.

The X and Y coordinates of the event are determined by associatedcircuitry 18 using as a main parameter the strength of the signalsgenerated by each PMT 14. The energy of the event is proportional to thesum of the signals, called the Z signal. Only Z signals within a givenrange are counted. A lead shield 20 surrounds the scintillation crystal12, the array of photodetectors 13 and associated circuitry 18 tominimize background radiation.

Generally, a collimator 22 is placed between scintillation crystal 12and the tissue. Commonly, collimator 22 is honeycomb-shaped, comprisinga large number of holes separated by parallel lead septa. The purpose ofcollimator 22 is to intercept and eliminate gamma photon emissions thatare not traveling in an accepted direction, i.e., parallel to the leadsepta. Also, as shown by FIG. 2, a glass 24 is generally placed betweenthe scintillation crystal 12 and the array of photodetectors 13.

A problem with prior art gamma cameras is that the surface of thescintillation crystal 12 which receives the gamma photon emissions ispolished uniformly to produce a uniform light response function (LRF)with respect to the location of a given PMT as shown by FIG. 2, andhence a uniform image. As a result, the PMT 14 directly over thedetected event (gamma ray interaction) receives most of the event'slight photons yielding less than optimum spatial resolution. If the PMTs14 are moved further away from the scintillation crystal 12 in order forthe photons from an event directly under the given PMT 14 to be seen bymore PMTs 14, the signal-to-noise ratio of the event degrades.Accordingly, for prior art gamma cameras, there is a trade-off betweensignal-to-noise ratio and spatial resolution.

One solution for this problem in the prior art is to place lightabsorbing shapes between the scintillation crystal and the PMTs foraltering the light response function of the scintillation crystal. Thismethod, however, causes the absorption of the light photons by the lightabsorbing shapes and therefore, degrades the energy resolution of thegamma camera.

Therefore, it is an aspect of the invention to provide a gamma camerayielding improved image quality with respect to spatial resolution forevents detected directly under a PMT center, i.e., by increasing thenumber of event's light photons the surrounding PMTs receive, andimproved signal-to-noise ratio without degrading the energy resolutionof the gamma camera.

SUMMARY OF THE INVENTION

With the foregoing and other aspects in view there is provided, inaccordance with the invention, a gamma camera having an array ofphotodetectors and associated circuitry for detecting and convertinglight energy to electrical energy. The gamma camera further includes ascintillation crystal positioned in proximity to the array ofphotodetectors for detecting gamma photon emissions and generating lightenergy.

At least one portion of at least one surface of the scintillationcrystal is polished differently than at least another portion foryielding a substantially different light response function for thegenerated light energy. That is, the scintillation crystal isnon-uniformly polished or has a non-uniform level of smoothness. Thenon-uniformly polished scintillation crystal improves signal-to-noiseratio and image quality with respect to spatial resolution. Thescintillation crystal is preferably sodium iodide-thallium activated(Nal(TI)) crystal.

The gamma camera in accordance with the invention further includes acollimator for intercepting and eliminating gamma photon emissions thatare not traveling in an accepted direction. A lead shield is alsoprovided and surrounds the scintillation crystal, the array ofphotodetectors and the associated circuitry for minimizing backgroundradiation. Further, a glass is positioned between the scintillationcrystal and the array of photodetectors.

The invention further provides a method for manufacturing a gammacamera. The method includes the steps of providing a scintillationcrystal wherein at least one portion of the scintillation crystal yieldsa different light response function for light energy generated by thescintillation crystal than at least another portion of the scintillationcrystal; providing an array of photodetectors having associatedcircuitry; and positioning the scintillation crystal in proximity to thearray of photodetectors.

The at least one portion of the scintillation crystal is polisheddifferently than the at least another portion for yielding the differentlight response function. That is, the scintillation crystal isnon-uniformly polished or has a non-uniform level of smoothness. Thenon-uniformly polished scintillation crystal improves signal-to-noiseratio and image quality with respect to spatial resolution. Thescintillation crystal is preferably sodium iodide-thallium activated(Nal(TI)) crystal. A glass is preferably positioned between thescintillation crystal and the array of photodectors.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will become more clearly understood from the followingdetailed description in connection with the accompanying drawings, inwhich:

FIG. 1 is a schematic illustration of a prior art gamma camera;

FIG. 2 is a schematic illustration showing gamma ray interactions with ascintillation crystal of a prior art gamma camera;

FIG. 3 is a schematic illustration of a polished surface of ascintillation crystal in accordance with the present invention;

FIG. 4 is a schematic illustration showing gamma ray interactions withthe scintillation crystal shown by FIG. 3; and

FIG. 5 is a schematic illustration of a gamma camera in accordance withthe present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 3, there is seen an exemplary embodiment of ascintillation crystal 100 for a gamma camera in accordance with thepresent invention. The scintillation crystal 100 is preferably sodiumiodide-thallium activated (Nal(TI)) crystal.

The scintillation crystal 100 includes a large surface area 102 fordetecting gamma photon emissions. The scintillation crystal 100 alsoincludes another large surface area 104 opposite surface area 102 forbeing viewed by an array of photodetectors 13 via glass 24 (see FIGS. 4and 5). At least one of the surface areas 102, 104, and preferably, bothsurface areas 102, 104, includes a plurality of first areas A and aplurality of second areas B which are polished differently with respectto each other. That is, one or both of surfaces 102, 104 of thescintillation crystal 100 are non-uniformly polished or have anon-uniform level of smoothness. The array of photodetectors 13 ispreferably an array of photo-multiplier tubes (PMTs) having a pluralityof PMTs 14 as known in the art.

The different polishing or level of smoothness for the plurality offirst areas A and the plurality of second areas B yield a differentlight response function (LRF) or non-uniform LRF for each of the areasA, B with respect to each other (see FIG. 4). That is, the plurality ofareas A have a first light response function and the plurality of areasB have a second light response function which is different from thefirst light response function.

Even though two different light response functions are describedhereinabove as being generated by the scintillation crystal 100, it iscontemplated that three or more different light response functions canbe generated by the scintillation crystal 100, if the scintillationcrystal 100 is polished three or more different ways as shown by FIG. 4(three different polished areas or areas having different levels ofsmoothness on at least one surface 102, 104 produce three different ornon-uniform light response functions).

The light response functions describe or define the propagation paths ofthe light energy generated by the scintillation crystal 100. The lightresponse functions can be characterized as being broad (LRF_1 in FIG.4), where the light energy is viewed, for example, by three or more PMTs14. The light response functions can also be characterized as beingnarrow (LRF_2, LRF_3 and LRF_4 in FIG. 4), where the light energy isviewed, for example, by one or two PMTs 14.

It is preferred that the plurality of areas A are substantially alignedwith a respective central axis of a PMT 14 of the array ofphotodetectors 13 and yield a broader light response function forimproving spatial resolution. Further, it is preferred that theplurality of areas B are not substantially aligned with a respectivecentral axis of a PMT 14 of the array of photodetectors 13 and yield anarrower light response function for improving the signal-to-noiseratio. As such, in the preferred embodiment, the plurality of firstareas A are polished more or made more smoother than the plurality ofsecond areas B.

FIG. 5 is a schematic illustration of a gamma camera in accordance withthe present invention and designated generally by reference numeral 500.The gamma camera 500 includes the same components as gamma camera 10 ofthe prior art and illustrated by FIG. 1. However, gamma camera 500includes the inventive scintillation crystal 100 for providing two ormore different light response functions for the generated light energy.

The invention further provides a method for manufacturing a gamma cameracomprising the steps of providing a scintillation crystal wherein atleast one portion of the scintillation crystal yields a different lightresponse function for light energy generated by the scintillationcrystal than at least another portion of the scintillation crystal asdescribed above. The method further provides the steps of providing anarray of photodetectors having associated circuitry and positioning thescintillation crystal in proximity to the array of photodetectors asshown by FIGS. 4 and 5.

The at least one portion of the scintillation crystal is polisheddifferently than the at least another portion for yielding the differentlight response function as described above. That is, the scintillationcrystal is non-uniformly polished or has a non-uniform level ofsmoothness. The non-uniformly polished scintillation crystal improvessignal-to-noise ratio and image quality with respect to spatialresolution.

The described embodiments of the present invention are intended to beillustrative rather than restrictive, and are not intended to representevery embodiment of the present invention. Various modifications andvariations can be made without departing from the spirit or scope of theinvention as set forth in the following claims both literally and inequivalents recognized in law.

1. A gamma camera for detecting gamma photon emissions and generatingelectrical energy comprising: an array of photodetectors and associatedcircuitry for detecting and converting light energy to electricalenergy; and a scintillation crystal positioned in proximity to saidarray of photodetectors for detecting gamma photon emissions andgenerating said light energy, wherein at least one portion of at leastone surface of said scintillation crystal yields a substantiallydifferent light response function for said generated light energy thanat least another portion of said scintillation crystal.
 2. The gammacamera according to claim 1, wherein said at least one portion of saidscintillation crystal includes a plurality of uniformly polished areas,and wherein each of said plurality of uniformly polished areas issubstantially aligned with a respective central axis of a photodetectorof said array of photodetectors.
 3. The gamma camera according to claim1, wherein said at least one portion of said scintillation crystalincludes a plurality of uniformly polished areas, and wherein each ofsaid plurality of uniformly polished areas is positioned such that it isnot substantially aligned with a respective central axis of aphotodetector of said array of photodetectors.
 4. The gamma cameraaccording to claim 1, wherein said at least one portion of saidscintillation crystal includes a first polished area of saidscintillation crystal and said at least another portion of saidscintillation crystal includes a second polished area of saidscintillation crystal, and wherein said first and said second areas arepolished differently to yield different light response functions forsaid generated light energy.
 5. The gamma camera according to claim 1,further comprising a collimator for intercepting and eliminating gammaphoton emissions that are not traveling in an accepted direction.
 6. Thegamma camera according to claim 1, wherein said scintillation crystal issodium iodide-thallium activated (Nal(TI)) crystal.
 7. The gamma cameraaccording to claim 1, further comprising a lead shield surrounding saidscintillation crystal, said array of photodetectors and said associatedcircuitry.
 8. The gamma camera according to claim 1, further comprisinga glass positioned between said scintillation crystal and said array ofphotodetectors.
 9. An improved scintillation crystal for a gamma cameraof the type comprising an array of photodetectors and associatedcircuitry for detecting and converting light energy to electricalenergy, a collimator for directing gamma photon emissions towards saidscintillation crystal, and a lead shield surrounding said scintillationcrystal, said array of photodetectors and said associated circuitry, theimproved scintillation crystal comprising: at least one portion yieldinga different light response function for light energy generated by saidscintillation crystal than at least another portion of saidscintillation crystal.
 10. The improved scintillation crystal accordingto claim 9, wherein said at least one portion of said scintillationcrystal includes a plurality of uniformly polished areas, and whereineach of said plurality of uniformly polished areas is substantiallyaligned with a respective central axis of a photodetector of said arrayof photodetectors.
 11. The improved scintillation crystal according toclaim 9, wherein said at least one portion of said scintillation crystalincludes a plurality of uniformly polished areas, and wherein each ofsaid plurality of uniformly polished areas is positioned such that it isnot substantially aligned with a respective central axis of aphotodetector of said array of photodetectors.
 12. The improvedscintillation crystal according to claim 9, wherein said at least oneportion of said scintillation crystal includes a first polished area ofsaid scintillation crystal and said at least another portion of saidscintillation crystal includes a second polished area of saidscintillation crystal, and wherein said first and said second polishedareas are polished differently to yield different light responsefunctions for said generated light energy.
 13. The improvedscintillation crystal according to claim 9, wherein said scintillationcrystal is sodium iodide-thallium activated (Nal(TI)) crystal.
 14. Amethod for manufacturing a gamma camera comprising the steps of:providing a scintillation crystal wherein at least one portion of saidscintillation crystal yields a different light response function forlight energy generated by said scintillation crystal than at leastanother portion of said scintillation crystal; providing an array ofphotodetectors having associated circuitry; and positioning saidscintillation crystal in proximity to said array of photodetectors. 15.The method according to claim 14, further comprising the step ofsurrounding said scintillation crystal, said array of photodetectors andassociated circuitry with a lead shield.
 16. The method according toclaim 14, further comprising the step of providing a collimator inproximity to said scintillation crystal and opposite said array ofphotodetectors.
 17. The method according to claim 14, wherein the stepof providing a scintillation crystal comprises the step of polishingsaid at least one portion of said scintillation crystal for yieldingsaid different light response function for light energy generated bysaid scintillation crystal than said at least another portion of saidscintillation crystal.
 18. The method according to claim 17, whereinsaid at least one polished portion of said scintillation crystalincludes a plurality of uniformly polished areas, and wherein each ofsaid plurality of uniformly polished areas is substantially aligned witha respective central axis of a photodetector of said array ofphotodetectors.
 19. The method according to claim 17, wherein said atleast one polished portion of said scintillation crystal includes aplurality of uniformly polished areas, and wherein each of saidplurality of uniformly polished areas is positioned such that it is notsubstantially aligned with a respective central axis of a photodetectorof said array of photodetectors.
 20. The method according to claim 17,wherein said at least one portion of said scintillation crystal includesa first polished area of said scintillation crystal and said at leastanother portion of said scintillation crystal includes a second polishedarea of said scintillation crystal, wherein said first and said secondpolished areas are polished differently to yield different lightresponse functions for said generated light energy.